An indium phosphide (InP)-based heterojunction bipolar transistor (HBT) is useful in applications where high-frequency operation and relatively high breakdown voltage, on the order of 10 to 20 volts, is desirable. The approximate relationship fMAX=√{square root over (fT/8πRbCc)}, where fT is the current-gain-bandwidth product, or cutoff frequency, Rb is the base series resistance and Cc is the collector-base capacitance, shows that as fT increases, the maximum operating frequency fMAX increases. The relationship is approximate because it is based on a simple lumped-element model of the transistor. Actually, the base series resistance Rb and collector-base capacitance Cc are distributed. More accurate expressions are algebraically complex and would obscure, rather than illuminate, the points that this approximate expression is used to make.
The breakdown voltage of a p-n junction is the reverse voltage beyond which a small increase in voltage causes a large increase in current. There are two definitions of breakdown voltage in a bipolar transistor. The voltage BVCEO is the transistor breakdown voltage measured with the base terminal open. The voltage BVCBO is the breakdown voltage of the base-collector junction measured with the emitter terminal open. In an indium phosphide (InP) HBT with gallium arsenide antimonide (GaAsSb) as the base material, BVCBO is limited by what is referred to as the “Zener” tunneling of electrons from the valence band of the base to the conduction band of the collector of the HBT. The current thereby produced is referred to as the “tunneling current.” It is generally desirable to maximize the voltage at which this tunneling current flows.
A GaAsSb/InP HBT exhibits superior high-frequency (high fT and fMAX) performance. The GaAsSb base forms what is referred to as a “type-II” band lineup with the InP collector. The band lineup is ideal in many respects because the conduction band offset improves electron transport from the base to the collector and the valence band offset minimizes hole transport from the base to the collector. The type-II band lineup leads to a desirable “knee” turn-on characteristic of the HBT. A good knee turn-on characteristic allows operation at a relatively low collector voltage and is important for the efficient operation of many circuits into which the HBT will be incorporated.
GaAsSb/InP HBTs have the above-described advantages but also a breakdown voltage insufficient for a number of potential applications. What is needed therefore is a GaAsSb/InP HBT with the above-described advantages, but with an increased breakdown voltage.